I looked into this a while back, and for my situation it wasn't worth it. My thought was a tube amp for mids/trebble and a 200 Wpc SS amp for bass, which I had. I tried feeding both amps from the pre, so both reproducing the full bandwidth, and found no material benefit. BTW, my tube amp gas gain attenuators to match gains.
To do it right, I thought of adding a crossover between pre and amps, and had to take the speakers' passive crossover out (the bass and mids/hi). That was too much a hassle/risk and decided to leave it alone. Some people have speakers that make taking that crossover out much easier, and from a theoretical point of view actively biamping still makes sense to me...so might give it another try in the future.
Yet this configuration requires an additional crossover, an additional pair of ICs, 2 additional power cords...when you add it up I can't help but wonder how a more expensive amp driving full bandwidth would compare!
Hence, the dilema. Thanks for allowing me to share my confusion! :-)
cheers